Abstract
Novel nanoporous hydrogel beads were obtained by one-pot synthesis, by cross-linking polyvinyl alcohol with glutaraldehyde and metal ions like zirconium, iron, and copper. These cylindrical beads had a porous morphology with the metal ion content of 11-14%. The beads exhibited a pH-independent, highly selective fluoride removal efficiency of ∼98% in simulated 10 mg L−1 fluoride solution, with negligible interference from HCO3−, PO43− and SO42−. Electrostatic interaction, hydrogen bonding and pore diffusion governed the mechanism of adsorption as ascertained by X-ray diffraction, differential scanning calorimetry, Raman and Fourier transform-infrared spectroscopy. The adsorption data fitted well in the Langmuir adsorption isotherm and predominantly obeyed pseudo-second-order kinetics accompanied by intra-particle diffusion. The maximum Langmuir adsorption capacity of hydrogel beads was 136.37 mg g−1, 130.21 mg g−1 and 74.96 mg g−1 for PVA-zirconium, PVA-copper-zirconium, and PVA-iron-zirconium respectively. The adsorption process was exothermic and spontaneous with ΔG° ranging from −37.0 kJ mol−1 to −52.0 kJ mol−1. The hydrogel beads had an excellent regenerability with ∼97% fluoride removal efficiency even after 10th adsorption-desorption cycle. The hydrogel beads reduced the fluoride concentration below 0.2 mg L−1 in the groundwater samples, making them promising adsorbents for real-time application in fluoride endemic regions.
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